Cutting Stack Effect Down to Size

By DONALD E. ROSS, FASHRAE

Thu, 2011-12-01 12:00

Editor's note: The following is adapted from the book HVAC Design Guide for Tall Commercial Buildings, published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. To purchase the book, go to http://bit.ly/ashraeross

A condition that exists in a tall building when outside temperature is significantly lower than the temperature of the spaces in the building is called stack effect. Stack effect is the phenomenon in which a tall building acts as a chimney in cold weather, with the natural convection of air entering at the lower floors of the building, flowing through the building, and exiting from the upper floors.

The cause of stack effect is the difference in density between the cold, denser air outside the building and the warm, less dense air inside the building. The pressure differential created by stack effect is directly proportional to building height, as well as to the differential between the warm temperature inside the building and the cold temperature outside the building.

When the temperature outside the building is warmer than the temperature inside the building, the stack-effect phenomenon is reversed. This means that in very warm climates, air will enter the building at the upper floors, flow through the building, and exit at the lower floors. This downward flow of air is known as reverse stack effect. The cause of reverse stack effect is the same in that it is caused by differences in density between the air in a building and the air outside a building, but in this case the heavier, denser air is inside the building.

While reverse stack effect would seem to be a problem in tall buildings in warm climates, this usually is not the case. The reason is that the difference between temperatures inside and outside a building and the resultant difference in density in warm climates is significantly less than the difference in temperatures inside and outside the building in very cold climates.

There is a neutral pressure level (NPL) in any building. This is the point at which the interior and exterior pressures are equal at any given temperature differential. The location of the NPL in any building is governed by the actual building, the permeability of its exterior walls, the internal partitions, and the construction and permeability of stairs and the shafts, including elevator shafts and shafts for ducts and pipes. Also influencing NPL are air-conditioning systems, with exhaust systems tending to raise NPL in a building (thereby increasing the portion of the total pressure differential experienced at the base of the building) and any excess of outside air over exhaust air in supply-air-conditioning systems tending to lower the neutral pressure level in the building (thereby decreasing the portion of total pressure differential experienced at the base of the building).

Figure 1 depicts the flow of air into and out of a building when the outside temperature is cold (stack effect) and when the outside temperature is hot (reverse stack effect). Not shown is the movement of air up or down within the building that occurs as a function of stack effect or reverse stack effect condition. NPL is the point in the building elevation at which air neither enters nor leaves the building. The vertical movement of air within the building will occur in the shafts and stairs, as well as any other openings that exist at the slab edge or in vertical piping sleeves at various locations that are less than perfectly sealed.

Figure 1 also indicates that the movement of air into and out of the building increases as the distance from the NPL increases.

Practical Considerations of Stack Effect
The existence of stack effect in tall commercial buildings often presents major problems. The problems most frequently manifest themselves in difficulty getting elevator doors to close and difficulty heating lower levels of the building. The elevator doors’ failure to close properly can be attributed to the pressure differential across the doors, which, in turn, causes the doors to bind in their guideway to the degree that the closing mechanism for the doors does not generate sufficient force to overcome the binding effect.

The heating problems can be attributed to the substantial influx of cold air through the doors at the entrance level and across the outside wall of the building becuase of the permeability of the wall being higher than the design requirement of the specification of the wall. The heating problem can be so severe as to freeze water in sprinkler-system piping and in cooling coils, if chilled water is not circulated.

Ways to Minimize Stack Effect
Fortunately, there are steps that can and should be taken in the design process to minimize the potential problems that will develop through stack effect. The necessary steps must be taken by both the architect and the HVAC design engineer. The steps that can be taken involve minimizing the air leakage. While it is not possible to completely seal any building, through consideration of the normal points at which outside air can and does enter and move vertically through a building, the problem can be mitigated.

The points at which outside air will infiltrate a building include the entry doors to the building as well as doors that open to truck docks, outside-air intakes, exhaust louvers, overhangs with light fixtures located immediately above ground level and not properly sealed against leakage or provided with heat, and small fissures in exterior walls.

Internally, a building will allow the passage of air through fire stairs, elevator shafts, mechanical shafts for ducts and piping, and any other vertical penetrations that exist at the edge of the floor slab at the exterior wall or for pipes. All these are candidates for careful review to ensure, to the degree possible, that a tight exterior wall is constructed, closure of all shafts is provided, and the sealing of all penetrations is provided. Vestibules or air locks can be provided for loading docks, with good door seals on the doors to and from the loading dock.

The HVAC designer must include mechanical air-conditioning and ventilating systems that supply more outside air than they exhaust. This is true of all systems where, to ensure pressurization, a full air balance is used for the entire building, with a minimum of 5 percent more outside air than the combination of spill and exhaust air being provided at all operating conditions.

In addition, it is good design, and often required by code for smoke-control reasons, to have a separate system for the entrance lobby. Although not always required, this system, if provided, can be designed to operate in extreme winter outside-air conditions with 100-percent outside air. Under these circumstances, air will be used to pressurize the building lobby, which is a point of extreme vulnerability in overall efforts to minimize the harmful impact of stack effect.

Donald E. Ross, FASHRAE, is a retired partner in the New York City-based mechanical and electrical consulting engineering firm of Jaros, Baum, and Bolles. He had primary design responsibility for more than 200 office buildings, hotels, hospitals, laboratories, and other projects on five continents. He is a past president of the New York Association of Consulting Engineers and served as vice chairman for North America for the Council on Tall Buildings and the Urban Habitat. He is member of the National Academy of Engineering.